This invention relates generally to television receivers and is specifically directed to improving the apparent resolution of images displayed by television cathode ray tubes.
The sharpness and crispness of a television image is dependent on factors such as the bandwidth of the transmitted signal, the bandwidth of the receiver circuitry, and the resolution capabilities of the image reproducing device. In the case of television receivers using cathode ray tubes as image reproducing devices, the spot size of the scanning electron beam is an important parameter in determining overall resolution capability.
In most cathode ray tubes, the spot size of the electron beam increases significantly as the beam current is increased. Therefore, when a large black-to-white video transition occurs, i.e., when a TV image includes a white area immediately following a black area, the spot size of the beam grows concurrently with the increase in the luminance signal. To a television viewer, the overall effect of the growth of the spot size is that, in the case of a white stripe on a black field, for example, the edges of the stripe will appear to be blurred with the white area expanded because of the large spot size and the black area correspondingly reduced. In the case of commercial color television programming, the net effect of spot size variance with luminance transitions is a television image which is less crisp than is desirable. This is particularly true where the picture contains many highlights, in which case the spot size of the scanning electron beam may grow to as large as 1/4 inch and completely obscure some video detail.
In an attempt to improve the crispness of television images, the prior art has, for the most part, concentrated on improving the bandwidth of television circuitry, improving electron guns so as to produce electron beams having smaller spot size and including "peaking" circuitry in the luminance channels of the receivers to generate steeper luminance transitions.
Although most commercial television receivers do include video peaking, the peaking may produce overshoots at points of amplitude transitions in the luminance signals. Such overshoots may contribute to a lack of resolution by increasing the magnitude of the luminance transitions and causing an increase in the electron beam size at the peak of the transitions, thereby creating an undesirable degrading effect on the resolution of the displayed image.
The literature does disclose attempts to improve the horizontal resolution capabilities of cathode ray tubes by modulating the scanning velocity of an electron beam so as to give the effect of a crispened television image. Examples of such attempts are disclosed in U.S. Pat. Nos. 2,678,964 and 3,830,958, for example. Briefly, one of the methods disclosed therein consists of processing the luminance signal to develop therefrom a control signal which may correspond to a derivative of the luminance signal. The control signal is then used to alter the deflection of the scanning electron beam in a way which causes the scan velocity of the beam to vary in accordance with the control signal. The variance in the scan velocity of the beam can, as will be pointed out below, result in a reproduced video image which has sharper edges, particularly on large black-to-white or white-to-black transitions. The improvement in resolution which might be achieved by this approach to enhancing the edges of TV images, is not, however, considered to be of the degree required to justify the additional expense of implementing this idea in television receivers for consumer use.
The remarks above pointing out how spot size can adversely affect horizontal resolution capability are also applicable to vertical resolution capability. In fact, spot size is perhaps the most important limiting factor in vertical resolution of television CRT's (cathode ray tubes). Since a video signal may be at full amplitude at one point in the picture and at zero amplitude at corresponding points one line above or below that particular point, the system might be said to exhibit infinite bandwidth in the vertical direction. The vertical resolution of television systems is limited, however, by television camera optics, the resolution of the camera tube, and as pointed out above, the spot size of the CRT electron beam. Thus, an edge enhancement system which can counter the effect of large spot size to improve both vertical and horizontal resolution would be a distinct improvement in the presentation of television images.
Finally, a television receiver employing an image edge enhancement system would probably not require the type of peaking presently found in most commercial receivers. As pointed out above, peaking can contribute to the growth of spot size when large amplitude video transitions occur. Therefore, if peaking is to be employed in a receiver in which image edge enhancement techniques are used, the peaking should be optimized to avoid generating large amplitude peaking components which would tend to counter the effects of the edge enhancement system.
Thus, although the concept of image edge enhancement is old, particularly the concept of varying the scan velocity of a CRT electron beam to increase the sharpness or crispness of displayed video images, the concept has not, up until now, been embodied in a television receiver in a way which fully exploits its possibilities. A television receiver which takes full advantage of the possibilities inherent in scan velocity modulation to sharpen the vertical edges of television images, particularly along with an edge enhancement system for improving the sharpness of horizontal edges, and along with a video peaking system which complements image edge enhancement, would be a great improvement over present commercially available television receivers.